An antiserum for bombesin, a tetradecapeptide, was used to map the regional distribution of bombesin-like immunoreactivity (BLI) in the rat brain. Using immunocytochemical techniques, BLI was observed in discrete varicose nerve fibers. Cell bodies were observed in the preoptic, hypothalamic and mammillary nuclei and in the mesencephalon. Using a radioimmunoassay combined with microdissection methods, BLI was observed to be 30-fold greater in high regions than low regions. When brain extracts were fractionated using high-pressure liquid chromatography, one major peak of immunoreactivity was obtained which was distinct from substance P. Release of bombesin-like peptides from rat hypothalamic slices was investigated. Immunocytochemical studies of the opiate ligand, enkephalin, revealed the distribution of a population of large axonal-like processes throughout the rat forebrain. The mapping of the distribution of these axonal-like processes, in addition to perikarya of the forebrain, was undertaken.